This invention relates to a bearing apparatus which uses a fluid lubricant such as a lubricating oil, a magnetic fluid and, the like, and more particularly, to a bearing apparatus suitable for use in a polygon mirror drive motor for laser beam printers or a motor for magnetic disk equipments, VTR and the like, which motor is required to exhibit a high rotational accuracy with a minimum of shaft whirling.
Recently, high speeds and high accuracy rotation with less shaft whirling are required of drive motors of the above type in the aspect of highly minute images and highly densed memory. In particular, a clean bearing apparatus adapted for "high accuracy rotation" and free from oil contamination are desired in laser beam printers and magnetic disk equipment.
With respect to such demands, problems relating to rotational accuracy and to contamination of main machines are directly related to performances of a bearing apparatus. In this respect, conventional ball bearings have limitations on high speeds and accuracy of rotation as a result of the accuracy in working rolling members and inner and outer races. For this reason, fluid-lubricated plain bearings have been employed as bearings which are effective in high speed rotation and enabling "high accuracy rotation", and various improvements have been made on such plain bearings for use in motors of the above type.
In a plain bearing, a fluid lubricant film formed on sliding surfaces upon rotation of a shaft maintains a shaft and a bearing in non-contact condition to rotatably support the shaft, and oil or gas is used as a lubricant. Unlike a gas bearing, a plain bearing making use of a lubricating oil is not expected to involve low torque, but provides an oil film of high rigidity which attains "high accuracy rotation" with less shaft whirling. Accordingly, such plain bearing can be designed to be of a small diameter as compared with a gas bearing, thus enabling a realization of a compact motor. However, in a plain bearing making use of a lubricating oil, oil leakage is always problematic, and dispersion of oil during high speed rotation poses a problem in the practical use of polygon mirror drive motors and magnetic disk spindle drive motors.
To cope with this problem, a magnetic fluid bearing has been proposed for use in drive motors of the above type including permanent magnet and a magnetic fluid with the magnetic fluid having a sealing function and providing lubrication. The magnetic fluid is formed by treating magnetic powders with a surface active agent and dispersing the same in a base oil. There are two types of magnetic fluid bearings classified in a basic construction. One type of magnetic fluid bearing retains a magnetic fluid on sliding bearing surfaces by magnetizing the same by a cylindrical-shaped permanent magnet, as disclosed in, for example, Japanese Patent Unexamined Publications Nos. 55-139559 and 59-147117. The other type of magnetic fluid bearing has a permanent magnet arranged at an end of the bearing and the permanent magnet and a permeable rotating shaft constitute a magnetic fluid sealing to have a magnetic fluid filled in a bearing section for lubrication, as disclosed in, for example, Japanese Patent Unexamined Publications Nos. 61-270520 and 60-88223. These two types of magnetic fluid bearings are intended for the prevention of dispersion of a magnetic fluid by magnetizing and providing the same with a sealing function, and are designed to increase magnetic flux density for sealing. In these prior bearings, a magnetic fluid is disadvantageously dispersed due to increased centrifugal forces upon rotation of high speeds if an amount of magnetic fluid as retained is slightly more than as required. Even if an amount of magnetic fluid as retained is appropriate, the magnetic fluid tends to be dispersed in the range of high rotational speeds due to cubical expansion produced by temperature rise since the magnetic fluid has a considerably large, thermal expansion coefficient as compared with constituent materials of the bearing, as described in the monthly magazine "Tribology" page 20, March 1988. In addition, there is a possibility of deterioration of a magnetic fluid when used at high temperatures. That is, as described in "Tribology" page 15, a surface active agent tends to separate from the magnetic powders when exposed to high temperatures, so that the magnetic powders will cohere to mar dispersion. In particular, in the case of a plain bearing, an eccentric load produced by centrifugal forces are applied on sliding surfaces to increase viscous shearing stresses on the lubricant film, unlike a magnetic fluid sealing. In addition, as rotational speeds are increased, a shear velocity becomes high to increase the density of generation of heat due to viscous friction, so that the surface active agent is separated from the magnetic powders to cause deterioration of the bearing performance. Therefore, thermal measures such as cooling are necessary in case a magnetic fluid is used as a sealing or lubricating fluid in high speeds.